A flood-level rim is the top edge of a fixture where water would spill over if the drain clogged, such as the rim of a sink or the overflow point of a toilet. Plumbing code uses this line as the reference for backflow protection, measuring the required air gap above it to keep dirty water out of the supply.
A flood-level rim is a precise term for a simple idea: it is the point on a fixture where water would start to spill over if the drain stopped working. On a sink, the flood-level rim is the top edge of the basin. On a bathtub, it is the top edge of the tub, or the overflow opening if one is lower. On a toilet, it is the rim of the bowl. In plain terms, it is the highest the water can rise before it runs onto the floor.
The term matters because plumbing code uses the flood-level rim as a reference line for protecting the drinking water supply. The danger it guards against is backflow, where dirty water from a fixture gets pulled backward into the clean supply because pressure dropped. The rule that prevents this is the air gap: code requires a water outlet, like a faucet spout, to sit a set distance above the fixture's flood-level rim. That open vertical space is measured from the flood-level rim up to the outlet.
Here is why the flood-level rim is the right line to measure from. If a sink drain clogs and the basin fills, the water rises only as high as the flood-level rim before spilling over the edge. As long as the faucet spout sits above that rim with an air gap, the rising dirty water can never reach the spout, so it can never be siphoned back into the supply pipe. For example, code sets the required air gap for a faucet at roughly twice the diameter of the outlet above the flood-level rim, which is why a normal faucet is mounted well above the sink edge rather than down at basin level.
Because the flood-level rim defines where overflow begins, it also comes up in rules about indirect waste and where drains can discharge. Any fitting or outlet that must stay clear of contaminated water is measured against this line. It is a small definition, but it anchors a large part of how code keeps the water supply safe.